This invention is a further development in the standing-wave linear charged particle accelerator art.
Since the earliest days of linear accelerator technology, beams of charged particles have been accelerated by the repeated application of electrical pulses at successive positions along the beam path through the accelerator structure. Various accelerator configurations have been developed to support an accelerating electric field along the beam path. The Sloan-Lawrence configuration, P. H. Sloan and E. O. Lawrence, 38 Physical Review 2021 (1931); the Alvarez cavity resonator configuration, L. W. Alvarez, 70 Physical Review 799 (1946); and the iris-loaded travelling-wave accelerator, E. L. Ginzton, W. W. Hansen and W. R. Kennedy, 19 Review of Scientific Instruments 89 (1948), are well-known. More recently, the side-cavity coupled accelerator configuration as described by E. A. Knapp, B. C. Knapp and J. M. Potter in an article entitled "Standing Wave High Energy Linear Accelerator Structures," 39 Review of Scientific Instruments 979 (1968), has found wide application.
The early standing-wave linear accelerators provided a succession of accelerating cavities and coupling cavities located one after the other along the length of the accelerator. Particles to be accelerated would travel first through an accelerating cavity and then through a coupling cavity in repeated succession throughout the length of the accelerator. Energy could be absorbed by the particles only in the accelerating cavities. Consequently, the coupling cavities had the effect of contributing to the overall length of the accelerator structure without imparting any accelerating force to the particles. It was subsequently realized that the coupling cavities could be disposed as side cavities away from the path of the particle beam. By positioning the coupling cavities away from the beam path, the overall length of the accelerator could be reduced. With the beam thus passing through accelerating cavities only, and not passing through coupling cavities, the energy gain of the beam per unit length of the accelerator could be increased. The side-cavity coupling technique thereby provided more efficient utilization of the radio-frequency power than had previously been possible. With side-cavity coupling, the beam was exposed to an accelerating electric field from which energy could be absorbed throughout the entire path length of the beam through the accelerator except for those portions of the beam path between adjacent accelerating cavities.
The energy absorbed by charged particles in an accelerating cavity is related to the time of flight of the particles across that cavity, so that an increase in the gap between the entrance and exit apertures of an accelerating cavity, can result in a decrease in the energy gain of the particles in that cavity. Present-day side-cavity coupled accelerator structures frequently employ drift tubes between adjacent accelerating cavities in order to optimize the energy gain in each accelerating cavity. Drift tubes are effective in correcting the adverse consequence of the lengthening of the accelerating cavities on the time of particle flight, and they also tend to concentrate the accelerating electric field within the immediate vicinity of beam path. However, drift tubes extend well into the accelerating cavities and typically occupy as much as one-third the overall length of the accelerator. Since the particles experience a substantially zero electric field intensity within the drift tubes, the particles do not acquire any energy during their passage through the drift tubes. Furthermore, drift tubes cause a high concentration of the electric field distribution at the entrance and exit apertures of the accelerating cavities, i.e., at the drift tube openings. This concentration of the electric field at the entrance and exit apertures of the accelerating cavities causes a reduction in the power level at which the accelerator can be operated without radio-frequency breakdown. The maximum permissible power level at which an accelerator may be operated without incurring radio-frequency breakdown determines the upper limit of the accelerating electric field that can be maintained along the beam path, and hence determines the maximum energy gain per unit path length of the beam through the accelerator. Maximization of the beam energy gain per unit accelerator length is especially important in applications such as radiation therapy where it is desirable to provide an accelerator structure that is as short as possible so that the accelerator structure can be rotatable in several planes within confined regions.